The present invention relates to a liquid crystal display device and to a method of manufacture thereof; and, more particularly, the invention relates to a liquid crystal display device which includes spacers for holding a fixed gap between a pair of substrates, which seal a liquid crystal layer therebetween, the spacers being constructed to decrease display irregularities, thus enabling an image display of high quality, and to a method of manufacture thereof.
As an example of display devices which are capable of producing a color display of high definition for notebook type computers and computer monitors, liquid crystal display devices have been popularly adopted. This type of liquid crystal display device is basically configured such that a liquid crystal layer is sandwiched in a gap defined between opposing faces of a pair of substrates, at least one of which is formed of a light transmitting glass plate or the like.
The liquid crystal display device is roughly classified into a type (a single matrix type liquid crystal display device) which turns on and off given pixels by selectively applying voltages to various electrodes defining pixels formed on a pair of substrates and a type (an active matrix type liquid crystal display device) which turns on and off given pixels by controlling various electrodes and pixel selecting switching elements which are formed on either one or both of the substrates.
The representative structure of the active matrix type liquid crystal display device lies in the fact that thin film diodes (TFD) or thin film transistors (TFT) are adopted as the switching elements formed on one of the substrates. Particularly, the liquid crystal display device which uses thin film transistors is thin and light-weight, and it can obtain a high quality compatible to the quality of a cathode ray tube, and, hence, the liquid crystal display device has been popularly used in display terminal monitors of various OA equipment.
The display method employed by the active matrix type liquid crystal display device is roughly classified into two types based on the difference in the driving method to be used. One display method is a method (a so-called vertical electric field method or a TN method) in which a liquid crystal layer is sandwiched between two substrates on which transparent electrodes are respectively formed, liquid crystal is operated by voltages which are applied to the transparent electrodes, thus modulating light which passes through the transparent electrodes and is incident on the liquid crystal layer so as to provide a display. Most products which are now popularly used adopt this method.
Another method is a method (a so-called lateral electric field method or an IPS method) in which liquid crystal is operated by an electric field which is formed between two electrodes formed on the same substrate and which is substantially parallel to a surface of the substrate, thus modulating light which is incident on the liquid crystal layer through a gap defined between the two electrodes, so as to provide a display.
In both types of liquid crystal display devices, it is necessary to maintain a gap between the pair of substrates which sandwich the liquid crystal layer, that is, a cell gap, at a given value. As means to hold the gap at the given value, minute beads (bead spacers) made of glass or resin have been popularly used conventionally. Since the bead spacers are scattered on a surface of one substrate by spraying or the like before the respective substrates are bonded to each other, the beads are distributed also in the pixel regions. Accordingly, transmitting light or reflection light passes through the bead spacers, and, hence, there has been a drawback that so-called leaking of light is generated, thus lowering the contrast.
Further, these liquid crystal display devices are manufactured in such a state that a large number of glass spacers are scattered on a surface of the substrate, and the spacers which are present in the pixel region become a cause of lowering of the image quality due to a defective orientation of the liquid crystal layer. Further, a coagulated spacer block or a non-uniformity of the scattering density becomes a cause of inducing a defective gap.
To overcome the drawbacks relating to the presence of the bead spacers, a structure has been proposed which fixedly forms columnar members between the pair of substrates as spacers. These spacers are referred to as columnar spacers (SOC) and are usually formed by applying a photosensitive region to the substrate and exposing, developing and baking the photosensitive resin by way of a mask on which a pattern for forming the columnar spacers is depicted. Particularly, the columnar spacers are arranged so as to avoid pixel electrodes, such as crossing portions between gate lines and drain lines, which are formed on substrates which face each other in an opposed manner.
With respect to the columnar spacers which are formed in this manner, although the gap between the opposing substrates is maintained, it is desirable that the columnar spacers have a fixed shape to hold the gap at a fixed value. Further, to support pressure which is applied to the columnar spacers at the time of manufacture or in the use of the liquid crystal display device, it is desirable that the diameter of the contact portions between the columnar spacers and the substrate is large.
In the above-mentioned step of baking the photosensitive resin, when the columnar spacers melt, irregularities arise with respect to the shape of the columnar spacers after the baking due to the irregularities in the shape and material of the spacers before baking, and the baking conditions. Particularly, with respect to the shape of the vicinity of a peak of the columnar spacer which is formed using a photo mask, a peak portion, that is, a surface which is brought into contact with the counter substrate, is rounded, and, hence, a portion where the columnar spacer and the substrate come into contact with each other becomes small, whereby irregularities in the cell thickness are liable to occur.
Further, the columnar spacer having such a round peak portion receives weight at one point compared with a columnar spacer having a flat peak portion, and, hence, there exists a possibility that the columnar spacer generates a deformation, such as warping, in the lateral direction.
As means to overcome this drawback, there has been proposed a technique to form peak portions (portions in contact with the counter substrate) on the columnar spacers of the liquid crystal display device having a recessed shape by exposing portions corresponding to the peak portions using a photo mask shielding the exposure portions from light partially or having a transmissivity which is changed continuously or in a step-like manner and their development thereafter, so as to keep the gap between the substrates uniform by enlarging the contact diameters where the peak portions come into contact with the counter substrate. Here, with respect to this type of construction, a technique disclosed in the following patent literature 1 is cited as an example.
[Patent Literature 1]
Japanese Unexamined Patent Publication 2002-229040